A substrate-type optical waveguide is in wide use, which includes a substrate having a surface with light guide paths. A substrate-type optical waveguide including a core made of a high refractive index material such as silicon and indium phosphide has a large refractive index difference between the core and the cladding, and has a strong light confining effect. The substrate-type optical waveguide can thus have a waveguide mode having a small mode field diameter.
The substrate-type optical waveguide typically receives input light to be introduced thereinto that has been outputted from an optical fiber or a laser diode (hereinafter abbreviated as “LD”). An optical fiber or a LD, however, outputs light having a mode field diameter that is typically larger than the mode field diameter of a waveguide mode for a substrate-type optical waveguide. Thus, to increase the coupling efficiency, it is important to, for example, increase the mode field diameter of a waveguide mode for a substrate-type optical waveguide in order to overcome the mismatch between the two mode field diameters.
The mode field diameter of a waveguide mode for a substrate-type optical waveguide may be increased by, for example, using a separation-type light guide path (see Patent Literature 1) or using an inverse-tapered light guide path as a core. Further, Non-Patent Literature 1 discloses, as a method for (i) rendering the mode field of a waveguide mode for a substrate-type optical waveguide flat and also (ii) increasing the mode field diameter, a method of using, as a core, two inverse-tapered light guide paths arranged adjacently to each other. The description below may use the term “mode conversion section” to refer to a light guide path having the function of increasing the mode field diameter of a waveguide mode such as the above light guide paths.